The present invention relates to a method of forming a resist pattern in a micro-lithography technique.
In recent years, as the semiconductor devices have been scaled down increasingly, the stability of micro-lithography technique and uniformity of the resist pattern have been required. In the micro-lithography processes, a resist is first applied on a semiconductor wafer for subsequent baking thereof before an exposure of the baked resist is carried out by use of a described photo-mask pattern such as reticle. Subsequently, the exposed resist is baked and the development thereof is then carried out to thereby form a photo-resist pattern. The photo-resist pattern is then used as a mask for etching the wafer before the used photo-resist pattern is removed. In such photo-lithography processes, the exposure system is most expensive, for which reason it is required to increase the rate of operation of the exposure system. In order to satisfy this requirement, the number of resist application systems is increased so that resist can be applied fast for subsequent exposure treatment by the exposure system without any rest. Such production management has the following difficulty. The number of wafers to be processed in one time is likely to be different for every system. In this case, it is conventional to temporarily store the wafers applied with resist in a wafer storage buffer provided between the resist-application system and the exposure system for subsequent supply thereof into the exposure system. It was found that the storage of the resist-applied wafer in the wafer storage buffer after the resist was applied to the wafer and before the resist-applied wafer will be exposed may cause a variation in scale of the resist pattern. The variation in scale of the resist pattern might depend upon the time of storage of the resist-applied wafer in the wafer storage buffer.
At present, a chemical sensitizing negative resist is generally used. The size of the chemical sensitizing negative resist was measured with reference to variation in storage time "Th" during which the resist-applied wafer is stored in the wafer storage buffer after the resist-applied wafer was baked until the stored resist is then placed into the exposure system, provided that a time from the resist exposure until the development is fixed. The wafer is stored in a standard atmosphere in the clean room. The exposure conditions are selected under which if the storage time is zero, the size of the resist pattern corresponds to the designed size. The result of measurement is shown in FIG. 1, from which it can be understood that as the storage time Th becomes long, then the resist pattern size becomes reduced until the storage time Th becomes 40 minutes.
The storage time from baking the exposed wafer until the development thereof relates to the degree of influence to the variation in size of the resist pattern. The resist is made of an organic material, for which reason an atmosphere in the resist-application system chamber is almost 100%-organic atmosphere but no moisture is present. The resist-applied wafer will have received a certain influence by humidity when the resist-applied wafer is pull out of the resist-application system. The moisture is taken into the resist in the moisture rich atmosphere. Particularly, the moisture content in the resist is certainly increased when the resist will have been stored in the wafer storage buffer.
If the size of the resist is once varied by absorbing the moisture in the storage time duration between baking the resist-applied wafer and the exposure thereof, then the size of the resist pattern is then also varied from the designed value. Particularly if the line width of the resist pattern is in the sub-half micron order or not more than 0.25 micrometers, then the chemical sensitizing resist is preferably used in the light of sensitivity and resolution. Notwithstanding, the chemical sensitizing resist is likely to vary in size largely by absorbing the moisture, for which reason the line width of the resist pattern is varied by only 0.05 micrometers. This however means that 20% variation in size thereof appears if the line width of the resist pattern is in the sub-half micron order or 0.25 micrometers.
Against the above problems, it is required to do a strict management of the temperature and humidity around the resist. A method of maintaining the resist-applied wafer at predetermined temperature and humidity is disclosed in the Japanese laid-open patent publication No. 4-326509. A central control system or a process control box is provided on the resist process line for controlling the temperature and humidity around the resist-applied wafer in the stand-by state in the wafer storage buffer. The Japanese laid-open patent publication No. 4-326509 does not mention how to control the temperature and humidity around the resist-applied wafer stored in the wafer storage buffer, for which reason it is difficult to apply the above prior art practically. Normally, the clean room is maintained at a temperature in the range of 23-25.degree. C. and at a humidity of about 45%. If the resist-applied wafer is placed in the clean room for the purpose of storage thereof, then the temperature and humidity are well controlled and maintained stable. Accordingly, it is considered that variations in temperature and humidity of the atmosphere is well suppressed in the clean room so that the variation in size of the resist pattern caused by variation in the content of the moisture in the resist is well suppressed even if the resist-applied is stored in the clean room. The above prior art needs to use large scale equipment such as the central control system and the process control box for controlling the temperature and humidity of the atmosphere around the resist-applied wafer stored in the wafer storage buffer, for which reason the above prior art is costly.
In the above circumstances, it had been required to develop a novel method of forming a resist pattern free from any variation in size thereof and applicable to a sub-half micron photolithography process without any increase in the equipment cost.
The present invention provides a method of storing a resist-applied wafer having been baked until an exposure thereof by use of a photo-mask, wherein the resist-applied wafer is placed in an atmosphere maintained at a humidity of not less than 80%. It is preferable that the resist-applied wafer is placed in an atmosphere maintained at a temperature not lower than the room temperature for at least about 10 minutes. It is also preferable that the resist-applied wafer is placed in an atmosphere maintained at a humidity of not less than 90% for at least about 5 minutes. It is possible to further store the wafer in a clean room before the exposure.
The present invention also provides a method of forming a resist pattern which comprises the following steps. A resist is applied on a wafer for subsequent baking the same. Subsequently, the resist-applied wafer is then stored in an atmosphere maintained at a humidity of not less than 80% until the resist-applied wafer is placed in an exposure system for exposure thereof by use of a photo-mask. A development of the exposed resist on the wafer is carried out to form a resist pattern. It is possible to further store the wafer in a clean room before the exposure. The above resist is preferably a chemical sensitizing resist.